Hot working of metals



United States Patent M HOT WORKING 0F WIETALS Roy A. Halversen, Dearborn, Mich. No Drawing Application December 4, 1953 p p s i l No. 396,323 7 cl. 29-424 This invention relates to=the hot working of metals, particularly to the hot. working of metals which oxidize easily in the atmosphere at'thetemperature required for working them. i ,The hot working of :metals is employed widely in industry to affect the crystalline; structure and, therefore, the properties of the metals, and to shape and form them into desired shapes and articles. Such operations generally include. rolling,,extruding, forging and the like, all of which are wellknown and need not be described here. Inasmuch as these ,operationsmust, of necessity, generally becarried'lout in contact with air, there is a great variation in the .ease and satisfaction with which they can be carried outbecause of the differing chemical reactivities of the diiferent metals. The more non-reactive metals, such as the-noble metals, offer little difliculty in carrying out either hot or coldworking operations. They do not oxidize readily in the air even at elevated temperatures and little or no difficulty is encountered in shaping, joining or otherwise working with them.

Many of the more common industrial metals, however, offer varying degrees ofdifiiculty in hot working operations. It is well known that iron oxidizes readily, particularly'when-hot, and the formation of the oxide as a nonadhere'nt scale onthe surface of the metal presents certaindifficulties in the hot working of the iron. An appreciable proportion of-the metal is lost as scale in such operations. .In the case of the more reactive metals, such as aluminum, magnesium titanium and the like, the formation of a thin, tightly adherentoxide film occurs rapidly on the surface of'the metal even at ordinary temperature and such films interfere seriously with the working of the metal even when the so-called cold working processes are employed. These ditficulties are exaggeratedz-when hot workingmethods are attempted. The oxides of these metals'generally have very high melting points-and protect the surface of the metal effectively against further oxidation at room temperature as long as the film is not disturbed. When such metals are subjected to hot working processes, however, the film is disturbed andibroken, additional quantities of oxide are formed immediately on freshly exposed metal surfaces and the oxide becomes dispersed within the body of the metal. The advantageous properties of the metal are thereby seriously impaired. These difficulties and adverse effects are so pronounced that the utilization of the very desirable but highly reactive metals, especially magnesium and titanium, "has been retarded greatly. Although methods for workingaluminum and,-mor'e recently, magnesium have been worked out and are used widely in industry, they are cumbersome, unsatisfactory and costly. Hot working and welding of magnesium, for example, is often carried dut 'inat atmospherefof helium, but this adds considerably to the cost of the process and contributes to the inconvenience of: the operation.

In-the case of titanium, these dilficulties are further accentuated because .of thehigh melting point of the metal and thefact that it isgenerally obtained from its ores in 2,893,114 Patented July 7, 1959 the form of a spongy mass which has a very large surface area per unit weight and which thus contains a correspondingly high proportion of oxide. Attempts to reduce the spongy mass to billets, bars and the like for further working have been unsatisfactory because of the unavoidable inclusionin the billet or bar of so high a proportion of oxide of titanium that the product is brittle and practically'unworkable using ordinary procedures. Attempts to use conventional fiuxing agents to convert the titanium oxide to a fluid slag which can be worked out of the sponge, but which at the same time forms a protective layer of the film on the metal surface and prevents it from further oxidation, have heretofore been unsatisfactory because of the generally high melting point of the slag formed and, also, because of the diffieulty of finding agents which combine readily with titanium oxides. As a result of the use of such conventional agents, the worked titanium generally contains such a high proportion either of the titanium oxide or of residual slag or fiux that little or no advantage is gained using such agents. It is well known that titanium has properties which should make it an extremely valuable industrial metal provided it could be worked readily after reduction of the ore. The value of such a process for working titanium, particularly for the hot'working of titanium, is unquestioned. It has now been found that the easily oxidizable metals, including titanium, aluminum, magnesium, boron, molybdenum, iron, nickel, cobalt and copper, can be hot worked readily by employing certain fiuxing agents, which are hereinafter described more fully, and that, when employing these agents, hot working operations, such as forging, hammering, rolling, drawing and the like, can be carried out without difficulty to obtain products formed from the metal which have a very low content of, or which are substantially free of, occluded oxide or flux. Due to the exclusion of other than negligible amounts of these agents from within the body of the metal, the advantageous properties of the metal can be realized fully and its industrial utilization promoted advantageously. In particular, the utilization of the more reactive metals especially titanium, to form desired shapes and articles having prop erties hitherto unrealizable when using conventional working procedures is thus made possible. The advantages of the invention will be further apparent as the description proceeds.

The term easily oxidizable metal as used herein means a metal which, in contact with air at the working temperature employed, becomes covered more or less rapidly with a film or layer of oxide which may be either adherent or non-adherent. These metals constitute the group consisting of magnesium and copper and the metals lying therebetween in the electromotive series of elements. The highly reactive metals above magnesium in the series, e.g. the alkali and alkaline earth metals other than magnesium, are so highly reactive that in contact with air they tend to become completely oxidized and thus are not generally employed in the metallic state. The term hot working as employed herein refers to the working of a metal at an elevated temperature belowits melting point.

The new fiuxing agents are metal salts having certain well defined essential properties relative to the metal being worked and the working temperature employed. The fiuxing salt employed has a melting point lower than, and a boiling point higher than, the particular working temperature employed so that it will exist in the liquid phase at the working temperature. In addition, the flux ing salt is of such a nature that, at the working temperature employed, it is oxidized by the oxygen of the air or by the oxide of the metal being worked to'give a volatile product containing the anion of the fiuxing salt and a nonvolatile product, generally an oxide, which contains the and; the flame usedto heat. the metal.

cation of the fluxing salt. In addition, the fluxing agent is chosen so that the non-volatile cation-containing oxidation product reacts readily with impurities in the metal, such; as the oxideofthe metal being worked and other metallic: oxides which. may be present in the metal, to ive a slag; which isliquid at. the. working temperature. Thefiuidnatureof the. slag may. result either from the fluidnatureof-thecompounds formed by the reaction of the, impurities or from. the liquid nature of, mixtures of these compounds with excessof the fluxing salt. Because= of theliquidnature of both the fluxing salt and of; the: resulting slag, they can be worked out of the metal; readilyby hammering, squeezing, rolling and the like; Also, be cause of their protective but liquid nature, theypermitf the ready; joining of the hot metal surfaces using, substantially conventional procedures. Although the fluxing-salt; employed is generallya single salt, such hose; available in commercial grades, it should be pointed-j out that mixtures of salts can be used, if desired, and that in certain instances advantage may be taken of the. property of salts of forming eutectic mixtures having a melting point lower than any of the single components. The invention-contemplates the use of such mixtures and eutectics. It should be pointed out, furtherg that itis often, advantageous to employ a mixture of fluxingsalts. whichwill form a slag which itself consists of an eutectic mixture, of salts. I

An added advantage accruing to the use of the new methodresults from the conversion of the extremely hard oxidesand. nitrides of the metals, which are usually present-inithe-metal prior, to the working operation, to softer compounds, e.g. to titanates, zincates, plumbates, phosphates-,cand; the like. The. presence of such softer compounds in the slippage planes of the. metal is much less objectionable than is thepresence of the hard oxides and nitrides. As a matter of fact, the retention of a small amount, of such softer compound in a hot worked metal maybe advantageous to serve as a dry lubricant in case; it; is desired to coldworlc the metal subsequently.

Although awidevariety'of fluxing salts can be used, those preferred are the halides of cadmium, tin, lead, zinc and copper. Among the halides, the chlorides, bromides and iodides are preferred because of their greater ease of oxidation as compared with the fluorides. In addition, the toxicity of fluorine compounds renders their use somewhat hazardous and for this reason they are less preferred than the other halides. It is noted, also, that the preferred halides are those of metals, particularly lead and zinc, having amphoteric properties, the metal itself thus being capable of existing in either at cation or an anion. It may be that this property contributes to the advantageous character of the halides of these metals asfluxing salts. It appears that either the metal being worked or the metal of the fluxing salt should have amphoterie properties. It is often the case that both meta-ls have amphoteric properties. At any rate, it has been found that the salts mentioned can be employed with entiresatisfaction as fluxing salts in the hot working of the easily oxidizable metals, including those given previously.

In carrying out the hot working operation, it is only necessary to cause the fluxing salt to be brought into contact with the hot metal surfaces and to thencarry out the working operation in conventional fashion. The fluxing salt can besprinkledor poured onto the metal surface either prior to or during heating. When gas heating is employed, the fluxing salt can be fed onto the surface ofgthe metal through the flame, e.g., through a conventional Welding torch equipped with a powder feed line sotha t-the fluxing salt passes through the flame and impinges on the metal surface. It is often satisfactory and desirable. to dissolvethe fluxing salt in a fuel, such as gasoline oralcohol, which is then burned in a torch Furthermore, mstcadrof thefluxingjsalts themselves, compounds which unite or react to form such salts duringthecombustion of the fuel can be dissolved therein. Thus, when gasoline containing lead tetraethyl and ethylene dibromide is burned, the lead appears in the combustion products as lead bromide and, if the flame is directed onto a metal surface, a sufficient amount of lead bromide is deposited on the surface to enable the metal to be hot worked with satisfaction according to the method herein described. It is apparent that a wide range of soluble metal compounds can be used for preparing such a fuel, including the metal alkyls, the metal saltsof organic acids and many other metalorgano compounds. The metalorgano compound can also contain the required halogen, or this can be added tothe fuel as.a separate compound which is soluble therein. When alcohol is used as the fuel, many of the fluxing salts, because of their solubility therein, can be dissolved directly in the fuel.

In certain instances the hotmetal can be dipped into the powdered or molten fluxing-salt and enough of the latter caused to adhere to the metal surface to enable the subsequent working operation to be carried out satisfactorily. Sufficient of the fluxing salt should be used to provide at all times enoughof'the salt-onthe surface of the metal to: insure the liquid natureofthe slag formed. It insufficient fluxing salt isused to combine with all of the metal oxide, the latter willstill be present as a high melting and infusible solid and cannot be worked out of the body ofthe metal; Furthermore, under such conditions there is lesstendency for the hot surfaces; of the. metal to remaincompletely covered and further oxidation ofthe. metal will take place rapidly. Theuse. of anexcessof the fluxing salt provides a ture of-the saltwith the slag formed which frequently has a melting point lowerthan either.

Following the hot working operation, itisoften desirable to clean the residual fluxing salt and slag from the metal surface to give thearticle a better appearance or to remove substances whichmight,.under certain conditions, lead to excessive corrosion of the surface of the worked metal when. exposed to moist air. In addition, the presence of suchresidual' substances may sometimes interfere with. subsequent finishing operations which it may be desired to carry out on the piece. Such removal of residual fluxing salts and: slag can be carried out conveniently by subjecting the piece while still hot-to the action ofaphosphorus oxide vapor, e.g., to the vapor of phosphorus pentoxide, substantially as described and claimed in co-pending application. Serial No. 364,209, filed June 28, 1953, now Patent No. 2,698,267. Such oxides generally, and preferably, contain at least a small proportion ofmc'taphosphoric acid. This treatment serves toconvert the metal compounds present on the surface of the metal to phosphorus-containing compounds which in tbemselvesprovide aprotective film on the metal surfaceand which either are not objectionable if' allowed to remain. on themetal surface or which can beremoved more easily than certain of theslags first formed, e.g. by washing, scouring andthe like. The effectiveness of the phosphates as corrosion prevention agents, and as lubricants for cold working operations, is well known and the procedure just described furnishes a ready means for depositing this particular type of film on the surface as an integral step in one modification ofthe hot-working process.

Certain advantages ofythe invention are apparent from the following. examples which; are. given by way of illustration only and are. not 'to be. construed aslimiting.

Example I A piece of brittle titanium wire about inch in diameter and /2 inch long was held: close to the surface of an iron anvil. T heiflameiof a blow. torch was directed onto-the piece of wire soasto heat it to approximately 250 degrees C. The. torchwasfed with gasoline containing lead tetraethyl and ethylene dibromidcso that the atmosphere surrounding the piece of wire contained finely divided lead bromide formed during the combustion of the leaded gasoline. The titanium wire was rotated-and pounded with a hammer in such a manner that it touched the anvil surface only during the actual blows of the hammer. The heating, rotating and hammering were-continued until the wire had been elongated to alengthof about 2% inches and the average diameter had been reduced accordingly. The wire was then allowed to cool. The final, product was much less brittle than the originaltrodand could be bent easily without breaking. The surface of the piece was dark reddish brown. This appeared to be due to the presence of a coating of complex lead and titanium compounds mixed with iron oxide from the anvil surface.

When a piece of the same wire was hammered in similar manner in a torch flame burning gasoline free from lead and bromine, the wire remained brittle and it was impossible to deform it without breaking to nearly the extent just described. Extensive oxidation of the metal also occurred.

Example 2 A piece of the same wire used in Example 1, A inch long, was heated and hammered as in Example 1 until it had an average diameter of about ,5 inch. It was then bent double and the hammering in the torch flame continued until a part of the doubled portion of the wire had been hammered into the form of a thin homogeneous sheet approximately ,4 inch thick. The sheet was flexible and could be bent many times without breaking. It was covered with a colored coating similar to that obtained in Example 1.

Example 3 A short length of brittle iron wire A inch in diameter, such as that ordinarily used in making coat hangers, was scraped clean with a steel knife and hammered in a leaded torch flame as in Example 2. The final sheet was covered with a layer of dark colored adherent flux. The sheet could be bent readily many times without breaking and with no cracking of the coating. A piece of the sheet was suspended partially immersed in percent sodium chloride solution for one hour. There was no sign of rusting or corrosion at the interface of the air and salt solution, indicating that the tightly adherent coating was highly effective in preventing corrosion of the sheet. By way of comparison, samples of the scraped but unheated steel wire broke readily when bent two or three times. A sample of the same scraped but unheated wire submitted to the same corrosion test described showed a definite ring of rust at the point of interface of the air and salt solution.

Example 4 A small porous billet of titanium was heated with cuprous chloride in a pan with a loose fitting lid for several minutes at about 600 to 700 degrees C. The billet was transferred quickly to a slightly cupped die under a steam hammer. The billet, which was 1.25 inches thick before hammering, was compressed by one blow of the hammer to a sheet about 6 x 3.5 inches and .375 inch thick. The density of the metal in the sheet was considerably greater than in the billet. Upon sectioning the sheet, it was apparent that a large proportion of the metal, particularly the portions of the sheet removed from its edges, was of a density suitable for most purposes.

diameter were heated in a flame formed by burning gasoline in a torch, the gasoline being free of metal and halogen compounds. The procedure of Example 1 was followed approximately using the fluxing salts given below at approximately the temperatures listed for each.

0 Approx. tempi (e) Cadmium chloride 600-750 The fluxing salt was applied, in someinstanciesfby sprinkling it on the rod during the hammering operation and, in other instances, by dipping the rod into the powdered salt after each several hammer blows.

After hammering, the pieces of rod, which had been elongated to about five times their original length, with a corresponding reduction in diameter, were each dipped into metaphosphoric acid and again heated and hammered briefly. The dark to dark brown surfaces of the metal pieces changed during this procedure to a bright metallic color. The same effect was observed by sprinkling a small amount of phosphorus pentoxide on the hot surfaces of some of the hammered rods and then hammering them again briefly.

I claim:

1. In the hot working of a metal selected from the group consisting of magnesium and copper and the metals lying therebetween in the electromotive series of elements, the method for preventing substantial accumulation of impurities in the metal which comprises the step of hot working the metal in an atmosphere of air and maintaining on the heated working surface of the metal a fluxing salt which has a melting point below, and a boiling point above, the working temperature and which is oxidizable under the working conditions to give a volatile product containing the anion of the fluxing salt and a non-volatile product containing the cation of the fluxing salt, the non-volatile product being reactive under the working conditions with the oxide of the metal being worked to form a liquid flux.

2. The method of claim 1 wherein the fluxing salt is selected from the group consisting of the chlorides, bromides and iodides of zinc, lead, copper, cadmium and tin, and mixtures thereof.

3. In the hot working of titanium, the method for preventing substantial oxidation of the titanium at the working surfaces which comprises the step of hot working the titanium in an atmosphere of air and maintaining on the heated surface of the titanium during working a fluxing salt which has a melting point below, and a boiling point above, the working temperature and which is oxidizable under the working conditions to give a volatile product containing the anion of the fluxing salt and a non-volatile product containing the cation of the fluxing salt, the non-volatile product being reactive under the working conditions with titanium dioxide to give a liquid flux.

4. In the hot working of a metal selected from the group consisting of magnesium and copper and the metals lying therebetween in the electromotive series of elements, the method for preventing substantial accumulation of impurities in the metal which comprises the steps of hot working the metal in an atmosphere of air and maintaining on the heated working surface of the metal a fluxing salt which has a melting point below, and a boiling point above, the working temperature and which is oxidizable under the working conditions to give a volatile product containing the anion of the fluxing salt and a non-volatile product containing the cation of the fluxing salt, the non-volatile product being reactive under the working conditions with the oxide of the metal being worked to form a liquid flux, and subsequently contact ing the hot metal surface with metaphosphorous acid to convert; remaining portions of the flux-ingsalt and of the 111115; to,phosphorus-containing compounds.

5". In the hot Working of titanium, the method-for pre venting substantial oxidation of the titanium at the workingsur-facelwhich comp-rises the steps of hot Working the titahium in. an atmosphere of air and applying to the heated-working surfacecofi the titanium, lead bromide as a fiuxingsalt to cover the. working surface with a'film of fluid-flux, and maintaining/the film on the Working surfaceduringvthec working operation.

6. The method of. claim 5 wherein the lead bromide is replaced with cuprous chloride as a fiuxing salt.

7. Themethodof claim 5 wherein the lead bromide is, replaced with zinc chloride asa fiuxing salt.

References Cited in thefileof-jhis patent- UNITED STATES PATENTS I Lang Nov. 4, 1913 Humberstone Aug; 6, 1935' Rodriguez June 30, 1936 Horowitz Sept. 22, 1942 Doerr June-2; 1953 Hall Apr. 13, 1954 Halversen Dec. 28, 1954 Slomin July 9, 1957' OTHER REFERENCES Page 152, American Machinist, Junell, 1951. 

1. IN THE HOT WORKING OF A METAL SELECTED FROM THE GROUP CONSISTING OF MAGNESIUM AND COPPER AND THE METALS LYING THEREBETWEEN IN THE ELECTROMOTIVE SERIES OF ELEMENTS, THE METHOD FOR PREVENTING SUBSTANTIAL ACCUMULATION OF IMPURITIES IN THE METAL WHICH COMPRISES THE STEP OF HOT WORKING THE METAL IN AN ATMOSPHERE OF AIR AND MAINTAINING ON THE HEATED WORKING SURFACE OF THE METAL A FLUXING SALT WHICH HAS A MELTING POINT BELOW, AND A BOILING POINT ABOVE, THE WORKING TEMPERATURE AND WHICH IS OXIDIZABLE UNDER THE WORKING CONDITIONS TO GIVE A VOLATILE PRODUCT CONTAINING THE ANION OF THE FLUXING SALT AND A NON-VOLATILE PRODUCT CONTAINING THE CATION OF THE FLUXING SALT, THE NON-VOLATILE PRODUCT BEING REACTIVE UNDER THE WORKING CONDITIONS WITH THE OXIDE OF THE METAL BEING WORKED TO FORM A LIQUID FLUX. 